Method and apparatus for blasting

ABSTRACT

An endless belt and a roller held in contact with said endless belt are turned at high rates of speed and at the same time an abrasive is delivered to the point of contact between said endless belt and said roller, whereby the abrasive is projected at a high rate of speed to impinge upon and blast the surface of an object subjected to cleaning which is placed at the plane of impingement. An apparatus for the purpose of this blasting comprises a blasting chamber incorporating therein an endless belt, a contact roller kept in contact with the endless belt, an abrasive feeder and an object subjected to blasting, apparatus such as, for example, a bucket elevator adapted to return the used abrasive to the abrasive feeder and a hopper. By use of this apparatus, the object is efficiently blasted and the used abrasive is cyclically used automatically.

United States Patent Nishio et al.

[ Nov. 25, 1975 METHOD AND APPARATUS FOR BLASTING Inventors: Yasuhiro Nishio, Tokyo; Hirosi Yonekubo, Kawasaki; Takeo Matuzawa, Shiojiri; Tosiki Hanamura, Shiojiri; Kunimoto Ishibayashi, Shiojiri, all of Japan Showa Denko Kabushiki Kaisha; Kyoei Kenmazai Company, Limited, both of Tokyo, Japan Filed: July 31, 1974 Appl. No.1 493,391

[73] Assignees:

Foreign Application Priority Data Aug. 9, 1973 Japan 48-88837 US. Cl. 51/9 R; 51/14 Int. C13"... B24C 1/00; B24C 3/14; B24C 5/06 Field of Search 51/9 R, 9 M, 14, 318, 319-321 [56] References Cited UNITED STATES PATENTS 11/1922 Townsend 51/9 R X 12/1953 Harper 5/1963 Straub 51/9 R 3,098,324 7/1963 Straub 51/9 R 3,243,920 4/1966 Hannum 51/9 R 3,328,925 7/1967 Hewitt 51/14 X 3,696,565 10/1972 Claeys 51/14 Primary ExaminerDonald G. Kelly Attorney, Agent, or Firm-Oblon, Fisher, Spivak, McClelland & Maier [57] ABSTRACT An endless belt and a roller held in contact with said endless belt are turned at high rates of speed and at the same time an abrasive is delivered to the point of contact between said endless belt and said roller, whereby the abrasive is projected at a high rate of speed to impinge upon and blast the surface of an object subjected to cleaning which is placed at the plane of impingement. An apparatus for the purpose of this blasting comprises a blasting chamber incorporating therein an endless belt, a contact roller kept in contact with the endless belt, an abrasive feeder and an object subjected to blasting, apparatus such as, for example, a bucket elevator adapted to return the used abrasive to the abrasive feeder and a hopper. By use of this apparatus, the object is efficiently blasted and the used abrasive is cyclically used automatically.

4 Claims, 9 Drawing Figures U.S. Patent Nov. 25, 1975 Sheetl0f6 3,921,336

U.S Patent Nov. 25, 1975 Sheet 2 of6 3,921,336

UQS. Patfint Nov. 25, 1975 Sheet30f6 3,921,336

US. Patent Nov. 25, 1975 Sheet40f6 3,921,336

Fig.5 D

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U.S. Patsnt Nov. 25, 1975 WEIGHT RATIO(/) Sheet 5 of 6 3,921,336

MESH

Fig.7

MESH

US. Patent WEIGHT RATIO (/o) WEIGHT RATIO Nov. 25, 1975 Sheet 6 f 6 MESH METHOD AND APPARATUS FOR BLASTING BACKGROUND OF THE INVENTION:

This invention relates to a method for blasting metallic, plastic, ceramic and other similar objects for thereby imparting satin finish patterns to metal surfaces, cleaning metal surfaces of adhering matter, scale and oxide film, exposing metal matrix, cleaning plastic and ceramic objects of adhering matter and removing them of fin, with the blasting effected by impacting an abrasive at a high rate of speed against said objects. It also relates to an apparatus which is used for practicing the method of blasting described above.

Of the present known blasting methods, those described below are typical exmaples.

A pneumatic method utilizes the expansion force of compressed air and a centrifugal method makes use of the centrifugal force generated by the rotation of an impellers. These methods are available for the purpose of blasting metallic objects.

Substances which are usable as abrasives for such methods are artificial abrasive, sand particles, shot (steel spheres measuring from 0.2mm to 2.8mm in diameter), glass beads and nutshell powder, for example.

The pneumatic method accomplishes the desired blasting of a given metallic object by utilizing a jet or forceful rush of such an abrasive as mentioned above ,which is produced by blowing said abrasive through a nozzle by means of compressed air. In this case, the abrasive is blown out through the nozzle at a high pressure. For this high pressure to be maintained, therefore, the diameter of the nozzle must be limited and, as a result, the zone covered by the jet of the abrasive is inevitably limited proportionally. This maintenance of high .pressure entails use of large volumes of air and accelerates the wear of the nozzle with the result that the cost of nozzle replacement is increased accordingly. There is another disadvantage in that the jet of the abrasive directed from the nozzle to the object subjected to blasting is reflected by the object to exert an adverse effect upon the velocity of the oncoming jet of abra- .sive. Further, since the jet coming out of the nozzle is formed of particles of the abrasive, the area of the object which is to be blasted by this jet is circular proportionately to the shape of the nozzle. The depth of abrasion caused in this circular area by the blasting is not uniform; it is the greatest at the center and the smallest on the periphery.

The centrifugal method effects the required blasting of a given object by rotating at a high rate of speed a rotary shaft which is disposed at the center of a rotary cylinder and provided with radially arranged blades and at :the same time delivering an abrasive to the center of :shortened to a great extent.

Further, the abrasive itself is disintegrated because of the vortex motion of air between the blades and the backward flow of air at the tips of the blades which are caused by the rotation of the rotary shaft and addition- 2 ally because of the impact of the blades in motion against the ambient air.

In the case of an abrasive which has a low specific gravity (as, for example, a specific gravity of not more than 3), the pressure of the air being reflected by the rotation of the blades acts upon and lowers the velocity with which the abrasive is advancing and consequently the efficiency of blasting is degraded.

The intensity of the blasting force being exerted on the surface of the object is not the same in the rotating direction of the rotary shaft and in the axial direction thereof and the blasting itself tends to be effected to the greatest depth at the center of the object.

One object of this invention is to provide a method for blasting which permits the surface of an object subjected to treatment to be blasted smoothly and uniformly without entailing any appreciable wear of the abrasive.

Another object of this invention is to provide a method for blasting which enables large volumes of objects to be blasted in a short priod of time.

It is also an object of this invention to provide an apparatus for blasting which does not wear away conspicuously and which permits the velocity of the jet of abrasive to be easily controlled depending on the particular kind of abrasive, the kind of the object being blasted and the purpose of the blasting.

Still another object of this invention is to provide an apparatus for blasting which enables desired blasting of given objects to be effected continuously with high efficiency.

SUMMARY OF THE INVENTION:

To attain these objects according to this invention there is provided a method for blasting an object subjected to blasting treatment, characterized by using an arrangement wherein an endless belt held taut in position by two spaced supporting rollers and a contact roller are disposed in such a relationship that said contact roller is held against said endless belt at a portion separated from the zones in which the endless belt comes into contact with its supporting rollers, causing the endless belt and the contact roller to turn at high rates of speed and at the same time feed an abrasive toward the point of contact between the endless belt and the contact roller, whereby the abrasive is caught and forwarded by virtue of the contact between the mutually approaching surfaces of the endless belt and the contact roller and then projected out at a high rate of speed so as to impinge upon and blast the surface of said object which is disposed in the path of the jet of abrasive. To effect the method of blasting according to this invention there is provided an apparatus characterized by comprising a blasting chamber having disposed therein an endless belt which is held taut in position by a roller and a movable adjusting roller, a movable contact roller which is held against said endless belt at a portion separated from the zones in which the endless belt comes into contact with its supporting rollers, a feeder which opens in the direction of the neighborhood of the point of contact between said endless belt and the contact roller and a means which is adapted to hold in position an object subjected to the blasting treatment, a hopper disposed above the feeder and adapted to accommodate the abrasive, a discharge outlet disposed at the bottom of the blasting chamber, a means such as, for example, a bucket elevator disposed between the hopper and the discharge outlet to the hopper and a driving means capable of turning the rollers at high rates of speed.

Other characteristics and other advantages of the present invention will become apparent from the following description which is made with reference to the accompanying drawings.

BRIEF EXPLANATION OF THE DRAWING:

FIG. 1 is a rough front sectional view of the apparatus of the present invention.

FIG. 2 is a front view of the important parts of one preferred embodiment of the unit for the projection of abrasive.

FIG. 3 is a partially cutaway side view illustrating the details of the roller and the adjusting roller which serve to hold the endless belt in position.

FIG. 4 is a top view of the contact roller.

FIG. 5 shows another embodiment in which the apparatus of this invention is used for blasting both work surfaces of an object.

FIG. 6 is a graph showing the condition of breakage observed to occur on an abrasive of fused alumina grains during its repeated use in blasting according to the method of this invention.

FIG. 7 is a graph showing the condition of breakage observed to occur on an abrasive of fused alumina grains during its repeated use in blasting according to the pneumatic method.

FIG. 8 is a graph showing the condition of breakage observed to occur on an abrasive of silicon carbide during its repeated use in blasting according to the method of this invention.

FIG. 9 is a graph showing the condition of breakage observed to occur on an abrasive of silicon carbide during its repeated use in blasting according to the pneumatic method.

DETAILED DESCRIPTION OF THE INVENTION:

This invention is described first with reference to one embodiment of the apparatus devised to effect the method of this invention.

FIG. 1 is a rough front view of one embodiment of the apparatus according to this invention.

In this diagram, 1 denotes a blasting chamber, 2 a roller, 3 an adjusting roller movable vertically in the illustrated position, 4 an endless belt held in position by the roller 2 and the adjusting roller 3 and 5 a contact roller held against said endless belt at a portion separated from the zones in which said endless belt 4 comes into contact with said rollers 2 and 3. Generally, the roller 2 has a rubber coating and is interlocked with a driving means. The endless belt 4 is revolved in the direction indicated by the arrow mark and has its tension adjusted by moving the position of the adjusting roller 3. The contact roller 5 is generally provided with a rubber coating and is possessed of an adjusting mechanism by which the rollers position can be moved as described afterward. The pressure which the contact roller 5 ex- .erts upon the endless belt 4 and consequently the interposing angle 0, which is formed between their surfaces can be adjusted by moving the contact roller 5 by means of this adjusting mechanism. The term interposing angle 0 as used herein refers to that angle which is formed between the two straight lines drawn to connect the two extremities of the zone of contact between the contact roller and the endless belt with the center of said contact roller. Denoted by 6 is a feeder serving to feed the abrasive. The opening of this feeder 4 is directed toward the nearer point of contact between the endless belt and the contact roller. Since the endless belt 4 advances, as already described, in the direction indicated by the arrow mark, the direction in which the abrasive is delivered from the feeder 6 is identical with the direction of the travel of the endless belt 4.

A means adapted to hold in position the object subjected to blasting treatment is denoted by 7. In the drawing, a belt conveyor laid through the blasting chamber functions as the means 7. The object 8 subjected to blasting treatment is shown positioned on the conveyor belt 7.

The endless belt 4 is revolved at a high rate of speed by means of the roller 2 and the adjusting roller 3 and, at the same time, the contact roller 5 is rotated so that both the endless belt 4 and the contact roller 5 turn at the same peripheral speed. Simultaneously a granular abrasive is supplied by the feeder 6. Particles of the abrasive are caught and carried forward between the mutually approaching surfaces of the contact roller 5 and the endless belt 4 and then projected in the direction of the object 8. Since the endless belt 4 and the contact roller 5 both travel at high rates of speed, the individual particles of the abrasive are tremendously accelerated by the momentum frictionally transferred from their surfaces. Consequently, the particles are projected at an extremely high rate of speed to impinge upon and blast the object. Even in the case ofa contact roller which is devoid of its own source of driving force, the particles of abrasive caught between the endless belt 4 and the contact roller 5 induce frictional resistance between the two surfaces, causing the contact rollers to rotate at the same peripheral speed as that of the endless belt 4. Consequently, there is manifested the same effect as when they are driven independently by their own driving means.

When the endless belt 4 is formed of a rubber belt or an abrasive belt and the contact roller has its surface covered with a rubber sheet or an abrasive fabric, possibility of breakage which may occur to particles of the abrasive while being moved forward between the two surfaces will be diminished to an extreme extent, and the particles are projected at the velocity generated because of the frictionally transferred force at the projecting angle 0 The term projecting angle 0 as used herein refers to that angle which is formed between the line along which the particles are projected and the surface of the object being blasted.

The velocity at which the abrasive is projected is determined by the peripheral speed of the endless belt and the coefficient of rolling friction of the particles of abrasive. The coefficient of rolling friction is determined by the diameter and shape of particles of abrasive, the rigidity and coarseness of the material of endless belt, etc.

The projecting angle 6 is determined by the diameters of the roller 2 and the contact roller 5 and the relative position at which the contact roller 5 is held in contact with the endless belt 4. This angle can be adjusted as required. The particles of abrasive which have been projected travel in a path of a fixed width 1 as illustrated in the diagram. The magnitude of this width 1 is variable with the interposing angle 0,. As regards the manner of contact between the endless belt 4 and the contact roller 5, if the aforementioned interposing angle 6 or the proportion of the portion of the circumference of the contact roller held in contact with the endless belt to its entire circumference is too small, then the particles of abrasive cannot be projected at a sufficiently high velocity. Any excess of this angle over its upper limit proves disadvantageous, and only adds to the wear of the belt and the roller. The desirable range of interposing angle (9 is from l5to 50.

When grooves are formed on the roller and the ad justing roller, they serve to protect the belt from adverse effects (such as flappling, deviation or break down of belt) which otherwise would be caused when irregularly reflected particles of abrasive possibly fly into the opening between the rollers and the endless belt.

Although in FIG. 1, the endless belt is illustrated as being advanced in a vertical direction and the path of the projected particles of abrasive as being directed downward, it is naturally optional that the path of the projected abrasive be obtained in any desired direction by properly changing the relative positions of the rollers and the endless belt.

For the method of blasting according to this invention, the desirable range of peripheral speed of the end less belt is from to 80 m/sec. More specifically, desirable results are obtained by selecting the peripheral speed of the endless belt from the following ranges, depending on the specific gravity of the abrasive particles.

For abrasive particles having a 60 to 80 m/sec specific gravity of not more than 4 For abrasive particles having a specific gravity of 4 or over 20 up to but not including 60/sec The particle size of the abrasive to be used has its 1 upper limit at 6mm. The desirable range of the particle size is from lu to 2mm. An abrasive having a particle size smaller than the lower limit of this range gives a rejectable blasting effect. When the particle size of a given abrasive is too large, the blasting produces a coarse surface and fails to give a cleaner finish.

The apparatus of this invention designed to effect the method of this invention enables the method described above to be carried out continuously with high effifor example, particle size finer than 140p,

A hopper intended to supply the abrasvie to the feeder 6 is denoted by the numeral 11. In addition to the fresh supply of abrasive which is delivered at intervals to the hopper 11, the portion of the abrasive which has completed its travel through the blasting chamber is recycled to the hopper. Denoted by 12 is a bucket elevator whose function is to convey back to the hopper the portion of the used abrasive from which excessively disintegrated abrasive particles have been removed by the sieve 10. The bucket elevator may be substituted by some other device which is suitable for the conveyance of the abrasive particles. The numeral 13 denotes a means adapted to blow the fine abrasive particles remaining upon the surface of the object which has undergone blasting. In the diagram, the belt conveyor serving as such means is moved in the direction indicated by the arrow mark.

FIG. 5 is a front view of another embodiment of the apparatus according to this invention: One plate given as an object 8 to be subjected to blasting is made to travel over a plurality of rollers arranged so as to serve as a means 7 for holding the object in a position desirable for blasting, so that the plate can be blasted on both surfaces.

This apparatus possesses the same devices as those illustrated in FIG. 1, except for another device for the projection of the abrasive. In FIG. 5, the numerals 2, 3', 4', 5 and 6' denote additional roller, adjusting roller, endless belt, contact roller and feeder respectively. The used abrasive which has been conveyed by the bucket elevator is supplied simultaneously to the feeder 6 and to the feeder 6. Use of the apparatus shown in FIG. 5 enables a plate material of large width to be blasted simultaneously on both surfaces.

Now, one embodiment of the device for the projection of abrasive will be described with reference to FIGS. 2, 3 and 4. In these diagrams, 6 denotes a feeder for the abrasive and 4 an endless belt which is held in position by a roller 2 and an adjusting roller 3 and revolved in the direction indicated by the arrow mark. The roller 2 is supported around a shaft 14 and this shaft 14 is provided at one end with a pulley 15 and is interlocked through the pulley with a driving means such as, for example, a motor. Denoted by 16 is a supporting frame. The roller 2 is fixed in position by means of the supporting frame 16 and a supporting base 17 which is attached to the supporting frame. The adjusting roller 3 is supported by a shaft 18, and the shaft 18 is supported by an adjusting roller supporting means 19. This adjusting roller supporting means 19 is connected via an air cylinder 20 to the supporting base 17. The numeral 27 denotes a tracking adjuster having threads cut on its outer face. The screw portion of the tracking adjuster engages an internally tapped hole formed in the adjusting roller supporting means 29. Desired adjustment of the position of the shaft 18 can be obtained by turning said screw portion. The adjusting roller 3 is integrally connected to the adjusting roller supporting means 19. Thus, it can be moved vertically on the supporting base 17 by means of the air cylinder 20. That is to say, the tension of the endless belt can be changed by suitably varying the distance between the roller and the adjusting roller. The contact roller denoted by 5 is attached by the medium of a contact roller supporting means 22 to a supporting frame 16. As illustrated, the contact roller supporting means is provided with oblong holes 23. The contact roller supporting means. 22 is attached to the supporting frame 16 by means of bolts 24. Since the oblong holes 23 have allowance on the righthand and lefthand sides with reference to the diagram, the position of the contact roller supporting means 22 can be moved within the longitudinal length of those oblong holes. A stationary base plate denoted by 25 is connected to the contact roller supporting means 22 by means of a bolt 26. The bolt 26 is passed through an internally tapped hole formed in the contact roller supporting means 22, with the screw on the bolt fitting the inner threads of said hole. When the bolts within the aforementioned oblong holes 23 are kept loose and the aforementioned bolt 26 is turned, the contact roller supporting means 22 can be moved within the aforementioned oblong holes 23 7 within the range of the motion allowed by the bolts 24. This means that the position of the contact roller can be changed by turning the bolt 26.

Generally, the method and apparatus according to this invention can be used for blasting objects of all kinds of substances. When they are applied to steel materials, they are as effective as the treatment using acid pickling. Where round steel bars are desired to be blasted, for example, a multiplicity of round steel bars are spaced by about 2mm and arranged in such a way that their axes are at right angles with the axes of the individual rollers and then they are moved in the axial direction with the abrasive projected thereto. The projected particles of abrasive, while in the process of passing the spaces intervening the bars, interfere with one another to give blasting uniformly to the entire surface of these bars. Similar effects are obtained in the case of materials such as stainless steel wires 5mm in diameter.

In the case of materials having a large diameter such as gas pipes 34mm in diameter, the projected particles of abrasive give blasting unevenly to the entire surface of these pipes.

In order to avoid such defect, it is advantageous to use a blasting apparatus in which four projecting machines are arranged around the blasting object, for example.

That is to say, it is possible to uniformly blast the entire surface of the pipe having a large diameter by arranging the pipe which is the blasting object in parallel with each roller of the four projecting machines, and causing the pipe to move in its axial direction while projecting particles of abrasive from the four projecting machines at a time.

By using the above-mentioned blasting apparatus, it is possible to easily blast special form bar materials such as shape steel for example.

Typical uses found for the apparatus of the present invention are enumerated below.

Surface treatment of aluminum articles in preparation for Teflon (tetrafluoroethylene resin) coating.

Grinding of projecting heads of rivets used on aircraft.

Removal of impurities from alloy ingots.

Removal of sediments accumulated on spike hangers in the aluminum electrolytic tank.

Removal of stain from repairing tools for use in cannery.

Processing of sheet or plate glass.

Smoothening of surface of drafting paper.

The blasting by the method and apparatus of this invention can be effected with unusually high efficiency and sufficient uniformity. The surface wear by this blasting is negligibly small. The amount of dust suffered to occur in the case of this invention is small as compared with any other known methods. Moreover, the blasting operation by this invention can be carried out continuously and automatically. In these and many other ways, this invention is advantageous.

Now, the effects derived from this invention will be demonstrated with reference to working examples, which are illustrative and are not limitative in any sense of the present invention.

EXAMPLE 1 In an apparatus having a construction like that shown in FIG. 1, a roller 2 measuring 200mm in diameter and 100mm in width was used and an aluminum plate was placed on a belt conveyor at a distance of 250mm from the center of the roller 2 and a distance of 380mm from the center of a contact roller 5, with the interposing angle 6 and the projecting angle 0 fixed respectively at 15 and An adjusting roller 3 was fixed at such a position that the line drawn to connect the centers of the roller 2 and the adjusting roller 3 fell at a perpendicular distance (L) of 225mm from the center of the contact roller 5. The roller 2 was rotated at the rate of 5750 rpm and an abrasive of fused alumina grains having a particle size of N0. 30 or an average particle diameter of 0.8mm was delivered from the hopper at the rate of 20 kg/minute. At this time, the actual working speed of the endless belt was about 3600 m/minute and, consequently, the contact roller was similarly rotated at the rate of about 3600 m/minute. This operation produced on the aluminum plate a beautiful satin finish pattern at the rate of 0.5 m lminute.

EXAMPLE 2 In an effort to demonstrate the present inventions feature that wear of the abrasive during repeated use in blasting is extremely low, the results of blasting performed by the method and apparatus according to this invention were compared with those obtained by the conventional pneumatic method.

An apparatus having a construction like that shown in FIG. 1 was used and an abrasive of fused alumina grains of particle size No. 24 (the curve of a continuous line in the graph of FIG. 6) was employed. An endless belt 100mm in width was revolved at the rate of 2000 m/minute. An ordinary steel plate was placed at a distance of 300mm from a contact roller, with the projecting angle 0 fixed at Under these conditions, blasting was performed with the same abrasive used in a total of five cycles. The particle size of the abrasive prior to the blasting and that after five cycles of blasting were as shown in FIG. 6.

In the graph of FIG. 6, the horizontal axis represents the graduation of sieve mesh and the vertical axis that of weight ratio (in percentage). The continuous line represents the values of the abrasive obtained prior to the blasting operation and the dotted line the values of the abrasive obtained after five cycles of blasting. Comparison of the curves reveals that the proportion of grains of small particle size increased only slightly as a whole, though the grains which initially had the minimum particle size of 420p. gave rise to grains of a particle size of the order of 70;; in consequence of blasting. This shows that even after five cycles of blasting the abrasive was not appreciably disintegrated.

COMPARATIVE EXAMPLE (WITH EXAMPLE 2) Under conditions necessary for obtaining the same surface finish, namely 4.0 kg/cm of air pressure, 300mm of distance to the object subjected to blasting treatment, 90 of projecting angle and 7mm of nozzle diameter, blasting was performed by the pneumatic method. The results obtained are shown in FIG. 7. In the graph, the continuous line and the dotted line have the same meaning as those in Example 2..

A look at the graph reveals that the abrasive was dis,- integrated more heavily as a whole than in Example 2.

EXAMPLE 3 Under the same conditions as those used in Example 2, blasting was performed by cyclically using a mixed abrasive of two SiC grades having different particle sizes No. 24 and No. 30. The results are shown in FIG.

1n the graph, the continuous line represents the values of particle size ratios existing prior to the blasting operation, the dotted line represents the values found after six cycles of blasting, the alternate one short and one long dash line represents the values found after 23 cycles of blasting and the broken line represents the values found after 50 cycles of blasting.

COMPARATIVE EXAMPLE (WITH EXAMPLE 3) Blasting was performed by faithfully repeating the procedure of Example 2, except for an abrasive which was made of SiC having a particle size of No. 24. The results are shown in FIG. 9. In the graph, the continuous line represents the values of particle size ratios existing prior to the blasting operation, the broken line represents the values found after six cycles of blasting and the alternate one short and one long dash line represents the values found after cycles of blasting.

In Example 3, grains having a particle size of 500p accounted for 50% even after 50 cycles of blasting. In contrast, in this case, grains having a particle size of 500p accounted for less than 10% by weight after only 10 cycles of blasting.

In Example 3, since the abrasive was a mixture of two SiC grades having different particle sizes No. 24 and No. 30, grains of 500g accounted for the initial maximum weight proportion. In the comparative example, since the abrasive was solely composed of one SiC grade of a particle size No. 24, grains of 710p. ac-

counted for the maximum weight proportion. Nevertheless, grains having a particle size of 500a in the comparative example fell to the order of after only six cycles of blasting. The effect of the present invention is evident from this comparison.

EXAMPLE 4 When blasting by the method and apparatus according to the present invention was carried out by using various kinds of abrasives, excellent results were obtained. These results are evaluated with reference to the results obtained by using the abrasive of fused alumina grains.

The blasting was performed in much the same way as in Example 1, with the efficiency of blasting and the loss of abrasive determined. The efficiency of blasting was rated in terms of the length of time required for obtaining a fixed surface finish, with the time required in the use of the abrasive of fused alumina grains taken as unity (I). The losss of abrasive was rated in terms of the amount of a given abrasive lost during one cycle of blasting, with the amount lsot in the case of abrasive of fused alumina grains taken as unity (1). The results are shown in the following table.

Table l Efficiency Abrasive of blasting Loss of abrasive Fused alumina 1.0 1.0 Silicon carbide 0.3 1.63 Garnet 1.0 2.07 (product of U.S.A.)

3.00 (product of China) Emery 1.4 1.92 White pig iron grit 1.7 0.14 Chromium slug 1.1 3.59 Nickel slug 1.4 4.33 Copper slug 2.9 2.03

Table l-continued Efficiency Abrasive of blasting Loss of abrasive Siliceous sand 1.4 4.63

Of the various abrasives listed above, fused alumina has a composition of 95.7% by weight (the same applies invariably to percentages given hereinafter) of Al- O 0.8% of SiO 0.5% of Fe O and 2.5% of TiO silicon carbide of 98.5% of SiC and 0.2% of free carbon, garnet of 38.0% of SiO 33.8% of FeO, 0.45% of TiO 2.23% of CaO, 0.21% of MgO, 20.95% of A1 0 and 1.22% of MnO. The term chromium slug refers to the slug which is produced in the refinement of ferrochromium and has a composition of 5.4% of CrO 28.7% of SiO 2.8% of FeO, 28.0% of A1 0 2.8% of CaO and 30.6% of MgO. The term "nickel slug refers to the slug which occurs in the refinement of ferronickel and has a composition of 0. 1 2% ofNiO, 6.13% of FeO, 31.6% of MgO, 56.77% of SiO 0.98% of A1 0 and 1.19% of CaO. Copper slug occurs in the refinement of copper and it has a composition of 4.1% of A1 0 37.7% of SiO 40.5% of FeO, 0.5% of Cu and 6.5% of CaO. Siliceous sand has a composition of 0.4% A1 0 and 98.8% of SiO EXAMPLE 5 Ordinary steel plates, aluminum plates and stainless steel plates were blasted in accordance with the same method and apparatus as those in Example 1 by using varying grades of fused alumina grains with different particle sizes as abrasives. The degrees of surface coarseness obtained on the metal plates are shown below. The numerical values given in the table represent highest depths of blasting.

In an apparatus having a construction like that shown in FIG. 1, an endless belt 350mm in width, a roller 250mm in diameter, an adjusting roller 250mm in diameter and a contact roller 250mm in width were disposed as illustrated and the endless belt was revolved at a peripheral speed of 3500 mm/minute and a steel plate was moved at the rate of 2.9 m/minute, with the magnitudes L (defined in Example 11), 0 (interposing angel) and 9 (projecting angle) varied as indicated below. With fused alumina used as the abrasive, the said copper plate was blasted to be stripped of hot scale. The results are shown below.

L 222mm 225mm 228mm 0, 18 l5.5 13 6 65 71 75 I (width of path of 29 m/m 22 m/m 20 m/m projected abrasive) mit adjustment of said distance from said roller, (c)

-continued an endless belt held in position by said roller (a) Resulof blastmg Good Fair and said adjusting roller (b), (d) a contact roller held in contact against said endless belt (c) at a point separated from the zones in which said end- When the blasting was performed under the same conditions as mentioned above but the peripheral speed of the endless belt was changed to 3300 m/minute, the results of blasting were as shown below.

less belt comes into contact with said rollers (a) and (b) and movable so as to permit adjustment of the strength with which the contact roller is kept in contact with the endless belt and (e) a feeder having an opening directed toward the point of contact Result of blasting Good Fair Fair between said endless belt and said contact roller and disposed so as to permlt the abrasive to be fed in the direction of the travel of the endless belt, we claim; 15 2. a means for accommodating the object subjected to blasting treatment, which means is separated by fixed distances from said roller and said contact roller and disposed opposite from said feeder with reference to said roller and said contact roller,

1. A method for blasting, which comprises: disposing l an endless belt supported in position by two rollers, (2) a contact roller held in contact against said endless belt at a point separated from I I the zones in which Said endless belt comes into 3. a blasting chamber containing therein said device contact with said supporting rollers, (3) an abrafor Projecting the abfasive and Said means for sive feeder the opening of which is directed toward lhg the ob ect, the point at which said contact roller and said end- 3 hooper dlsposed above 531d feeder and Provlded l b l reach h Starting point f Contact when with a means for delivering into the feeder its consaid endless belt is put to motion and (4) an object tents subjected to blasting treatment which is positioned a discharge Outlet formed at the bottom of the at a distance from the point at which the contact blasting Chamber and adapted to discharge the between said contact roller and said endless belt used abrasive from Within the blasting Chamber, terminates and causing said endless belt to revolve a means adapted to form a P leading from Said and said contact roller to rotate at substantially the discharge Outlet t0 aid hopper for thereby returnsame high rate of speed and in opposite directions, g to Said pp What has been discharged from and at the same time allowing the abrasive feeder Said d h ge utlet and to discharge th abrasive f r the b d liv i 7. adriving means for imparting high-speed rotations said abrasive between the mutually pp oa hin to the rollers and said endless belt, said rollers and surfaces of said contact roller and said endless belt, 5 Sa d be t be ng dri en at substantially the same rate whereby the abrasive is projected at a high rate of of speed and in opposite directions. speed by means of the momentum of the motion of 3. The apparatus of claim 2, wherein the means for said roller and said endless belt so as to impinge accommodating the object is a belt conveyor laid upon the object being treated. through the blasting chamber. 2. An apparatus for blasting, which comprises: 4. The apparatus of claim 2, wherein an additional 1. a device for projecting an abrasive, consisting of device for projecting the abrasive is disposed opposite (a) a roller, (b) an adjusting roller positioned at a from said device for projecting the abrasive.

distance from said roller and movable so as to per- 

1. A method for blasting, which comprises: disposing (1) an endless belt supported in position by two rollers, (2) a contact roller held in contact against said endless belt at a point separated from the zones in which said endless belt comes into contact with said supporting rollers, (3) an abrasive feeder the opening of which is directed toward the point at which said contact roller and said endless belt reach the starting point of contact when said endless belt is put to motion and (4) an object subjected to blasting treatment which is positioned at a distance from the point at which the contact between said contact roller and said endless belt terminates and causing said endless belt to revolve and said contact roller to rotate at substantially the same high rate of speed and in opposite directions, and at the same time allowing the abrasive feeder to discharge the abrasive for thereby delivering said abrasive between the mutually approaching surfaces of said contact roller and said endless belt, whereby the abrasive is projected at a high rate of speed by means of the momentum of the motion of said roller and said endless belt so as to impinge upon the object being treated.
 2. An apparatus for blasting, which comprises:
 1. a device for projecting an abrasive, consisting of (a) a roller, (b) an adjusting roller positioned at a distance from said roller and movable so as to permit adjustment of said distance from said roller, (c) an endless belt held in position by said roller (a) and said adjusting roller (b), (d) a contact roller held in contact against said endless belt (c) at a point separated from the zones in which said endless belt comes into contact with said rollers (a) and (b) and movable so as to permit adjustment of the strength with which the contact roller is kept in contact with the endless belt and (e) a feeder having an opening directed toward the point of contact between said endless belt and said contact roller and disposed so as to permit the abrasive to be fed in the direction of the travel of the endless belt,
 2. a means for accommodating the object subjected to blasting treatment, which means is separated by fixed distances from said roller and said contact roller and disposed opposite from said feeder with reference to said roller and said contact roller,
 3. a blasting chamber containing therein said device for projecting the abrasive and said means for accommodating the object,
 4. a hooper disposed above said feeder and provided with a means for delivering into the feeder its contents,
 5. a discharge outlet formed at the bottom of the blasting chamber and adapted to discharge the used abrasive from within the blasting chamber,
 6. a means adapted to form a path leading from said discharge outlet to said hopper for thereby returning to said hopper what has been discharged from said discharge outlet and
 7. a driving means for imparting high-speed rotations to the rollers and said endless belt, said rollers and said belt being driven at substantially the same rate of speed and in opposite directions.
 3. The apparatus of claim 2, wherein the means for accommodating the object is a belt conveyor laid through the blasting chamber.
 3. a blasting chamber containing therein said device for projecting the abrasive and said means for accommodating the object,
 4. a hooper disposed above said feeder and provided with a means for delivering into the feeder its contents,
 4. The apparatus of claim 2, wherein an additional device for projecting the abrasive is disposed opposite from said device for projecting the abrasive.
 5. a discharge outlet formed at the bottom of the blasting chamber and adapted to discharge the used abrasive from within the blasting chamber,
 6. a means adapted to form a path leading from said discharge outlet to said hopper for thereby returning to said hopper what has been discharged from said discharge outlet and
 7. a driving means for imparting high-speed rotations to the rollers and said endless belt, said rollers and said belt being driven at substantially the same rate of speed and in opposite directions. 